The video, taken on Thursday, later shows the man's body on the shore. The Egyptian army has so far not commented on the incident.

Officials from Hamas, the Islamist movement that rules the Gaza Strip, say he suffered from mental illness, though the father of the Gazan man has been quoted as saying he did not.

The hashtag "Why did they kill him?" has since spread on social media and several dozen Gazans demonstrated on Sunday to demand Egypt return the man's body to his family.

Gaza has been under an Israeli blockade since 2006, while its sole crossing with Egypt has been largely closed since the summer of 2013, when Egyptian president Mohamed Morsi of the Muslim Brotherhood was overthrown.

Cairo accuses Hamas, which is allied with the Muslim Brotherhood, of supporting attacks in Egypt.

The blockade and the closed Egyptian border means 1.8 million Gazans are effectively trapped inside the small enclave hit by three wars with Israel since 2008.